As computer chip device dimensions continue their evolution towards feature sizes below 100 nm, new liner materials and associated process technologies are needed to ensure viable diffusion barrier and adhesion promoter performance between the conductor and the surrounding regions of silicon-based and dielectric-based materials. These liners must possess mechanical and structural integrity, good conformality within aggressive device features, high conductivity to minimize plug overall effective resistance, and thermal, mechanical, and electrical compatibility with neighboring conductor and dielectric material systems. Most importantly, liner materials are expected to meet these stringent requirements at increasingly reduced thicknesses, in order to maximize the real estate available for the primary metal conductor within the continuously decreasing device dimensions. In particular, liner thickness is predicted to decrease from 20 nm for the 0.15 μm device generation, to less than 6 nm for its 0.05 μm in counterpart as noted in the International Technology Roadman for Semiconductors, 1999 Edition, Santa Clara, Calif., pp. 165.
These needs are further complicated by the on-going transition from aluminum-based to copper-based metallization schemes, and requires exploring new candidate liner materials that are chemically and thermodynamically more stable towards copper diffusion and migration than the presently used titanium/titanium nitride materials. These include binary compounds, such as tantalum nitride (TANx) and tungsten nitride (WNx), and ternary compounds such as tantalum-silicon-nitride (TaSixNy) and tungsten-silicon-nitride (WSixNy). See, M. Takayama et al., J. Vac. Sci. Technol., B 14, pp. 674 (1996); M. Uekubo et al., Thin Solid Films, 286, pp. 170 (1996); and K. Nakajima et al.,Appl. Surf. Sci., 117/118, pp. 312(1997).
Tungsten nitrides are refractory compounds with high densities and exceptional hardness and barrier properties. They are usually regarded as interstitial compounds, i.e., with a range of non-stoichiometric ratios of tungsten and nitrogen atoms. Even so, two simple stoichiometries are often represented, ditungsten nitride and tungsten nitride. Representative properties are as follows:
TABLE ACompoundDensity (g/cm3)Lattice ParameterW2N12.0-17.70.412WNsame as above0.289
Manufacturing technology for the production of tungsten nitride is accomplished by the nitriding of tungsten with nitrogen or ammonia, often in the presence of hydrogen at temperatures above 600° C. and under pressure. E. Markel et al., Kirk-Othmer Encyclopedia of Chemical Technology, “Nitrides,” 17, pp. 114 (1996). Manufacturing processes leading to tungsten nitride with more uniform properties and lower process temperatures are desirable since many potential applications for tungsten nitride films are temperature sensitive. For example, in microelectronic applications there is a need for a low temperature process for forming tungsten nitride, since such applications require consistent film properties in order to achieve uniform electrical properties and stress free coatings (uniform lattice parameters or amorphous structure) without damaging structures by exposure to high temperatures or corrosive by-products.
While WNx presents a potentially viable solution given its attractive properties as highly refractory material with excellent mechanical and chemical properties, there remains problems with achieving high conformality with low impurities. Since tungsten nitride can be deposited in amorphous form (see, B. Park et al., J. Electron. Mater., 26, pp. 1 (1997)), it is highly desirable for microelectronic applications, given that an amorphous film inherently has no grain boundaries. The lack of grain boundaries provides added stability towards metal migration by eliminating grain boundaries as a primary path for metal diffusion. Accordingly, prior work in the literature has successfully demonstrated the applicability of W2N as an effective barrier against copper diffusion at temperatures as high as 750° C. M. Uekubo et al., Thin Solid Films, 286, pp. 170 (1996).
Uekubo, noted above, has deposited tungsten nitride by reactive sputtering. Unfortunately, the application of sputtering techniques is limited by concerns over the ability to provide good conformality in sub-100 nm device structures.
Tungsten nitride has also been deposited by chemical vapor deposition (CVD) from tungsten tetrafluoride (WF6) and ammonia (NH3). S. Marcus et al., Thin Solid Films, 236, pp.330 (1993). While such CVD-based methods could provide viable step coverage in such aggressive topographies, thereby allowing the potential use of the same CVD liner technology in multiple generations of sub-100 nm microprocessor and memory products, inorganic CVD from WF6 and NH3 poses challenges. The difficulties include: (a) transport and handling concerns attributed to the high reactivity of the fluorinated WF6 source, (b) process concerns caused by potential gas phase particle generation in the reaction of WF6 and NH3 and (c) reliability issues pertaining to the possible inclusion of fluorine in the resulting WNx liner which is a fast diffuser in metals such as copper.
M. H. Tsai et al., Appl. Phys. Letters, 68, pp. 1412 (1996) also demonstrates a metal-organic CVD (MOCVD) from a single tungsten source, i.e., bis-(tert-butyl-imido)bis(tertbutylamido)tungsten having the formula ((t-BuN)2W(NHt-Bu)2). MOCVD from such single source precursors typically requires high processing temperatures, in excess of 450° C., with resulting film resistivities generally greater than 620 μΩ/cm. Higher resistivities result from significant thermal sensitivity of such single source prior art tungsten precursors for MOCVD. At temperatures of about 450° C. and higher, the hydrocarbon-based ligands in such sources tend to decompose readily, leading to film contamination with carbon and, as a result, higher resistivity.
Such recent attempts to generate WNx films consistent with microelectronic manufacturing have focused on methods utilizing tungsten hexafluoride as a precursor, with typical process windows for tungsten hexafluoride requiring temperatures greater than 450° C. as noted above with respect to Marcus et al. See also T. Nakajima et al., J. Electrochem. Soc., 134, pp. 3175 (1987). Alternately, plasma methods using tungsten hexafluoride have been used, but these still require temperatures greater than 350° C. See C. Meunier et al., Mater. Manufacturing Process., 13, pp. 415 (1998) and J. P. Lu et al., J. Electrochem. Soc., 145(2), pp. L21 (1998). Both the tungsten hexafluoride and plasma methods share the corrosive by-products of ammonium fluoride and hydrogen fluoride which cannot only react with tungsten nitride, but also with other metals and dielectrics which are typically part of microelectronic structures, for example, silicon, silicon dioxide, and aluminum. Fluorine is also a fast diffuser in copper. The direct erosion of silicon by tungsten hexafluoride has also been reported. D. Baxter et al., Chem. Mater., 8, pp. 1222 (1996).
Attempts to use organic-tungsten compounds to produce tungsten nitride films involve precursors which, under the best reported conditions invariably incorporate carbon into the films and have not shown low resistivities. M. Tsai et al., Appl. Phys. Lett., 68, pp. 1412 (1996). Dimethylamido-substituted tungsten compounds have been shown in model studies not to be stable enough to volatilize or undergo transport without vapor phase particle formation, which is inconsistent with the formation of uniform coatings. See, Baxter, above.
Furthermore, as device sizes continue their trend towards smaller features, tungsten nitride liner materials must provide the required performance at continuously reduced thicknesses in order to maximize space availability for the actual copper conductor. Predictions published in the International Technology Roadmap for Semiconductors-1998 Update indicate the need for liners with thicknesses below 1 nm for the 50 nm node. These trends require the development and optimization of manufacturing-worthy processes for the reliable and reproducible deposition of conformal ultrathin liners with atomic level controllability. As a result, work in the prior art has demonstrated that techniques such as atomic layer CVD (ALCVD) and atomic layer deposition (ALD) are viable candidates for incorporation in sub-tenth-micron semiconductor device fabrication flows. DiMeo, Jr., et al., U.S. Pat. No. 5,972,430; Suntola et al., U.S. Pat. No. 5,711,811; Suntola et al., U.S. Pat. No. 4,389,973; and Suntola, et al., U.S. Pat. No. 4,058,430.
These techniques are almost universally based on the principle of self-limiting adsorption of individual monolayers of source precursor species on the substrate surface, followed by reaction with appropriately selected reactants to grow a single molecular layer of the desired material. Thicker films are produced through repeated growth cycles until the desired target thickness is met. Unfortunately, the ALCVD approaches described in the prior art have not successfully reported the growth of WNx films with electronic grade quality for microelectronics applications, in part due to the lack of an appropriate metal-organic source chemistry which is amenable to the self-limiting adsorption in monolayer form on the substrate surface. In addition, the high chemical and thermal instability of many of the metal-organic tungsten source precursors used in the prior art can cause premature decomposition of the source chemistry upon contact with the substrate surface. The net outcome is the growth of highly contaminated films via a more conventional CVD-based thermolytic or pyrolytic approach, instead of the self-limiting, layer-by-layer, ALD approach. Even in the case of the inorganic tungsten hexafluoride source chemistry, there has not been a successful identification of an appropriate process window and/or intermediate precursor species that can successfully and reliably allow adsorption and decomposition in a layer-by-layer ALD mode.
Accordingly, there is a need in the art for a method for producing tungsten nitride and tungsten nitride films for use in a variety of applications, and in particular for microelectronic applications which provides good conformality, does not have difficulties in transport or handling, does not lead to significant gas phase particle generation from reaction of the precursor(s) used with the nitrogen source and which avoids inclusion of corrosive materials such as fluorine in the resulting tungsten nitride liner which may diffuse rapidly in metals in microelectronic applications such as copper. There is further a need for such a method which can produce tungsten nitride and tungsten nitride films with low or no carbon impurities in the resulting materials and which produce sufficient resistivities. There is also a need in the art for a method to produce films, including tungsten nitride films, of increasingly smaller thicknesses while retaining high conformality and uniformity.
There is further a need in the art to develop chemically-engineered, highly maleable tungsten source precursors for incorporation in atomically-tailored, interfacially-engineered, CVD processes for the deposition of highly conformal ultrathin tungsten nitride films, as thin as a few monolayers. Further, it would be desirable if these atomic-layer CVD (ALCVD) processes were able to demonstrate the necessary ability to chemically and structurally “nano-engineer” the substrate surface through tightly controlled interactions with the chemically-engineered source precursors or appropriate source precursor intermediates to allow sequential atomic layer by atomic layer growth of films such as tungsten nitride.